I don't think there's going to be only Na'vi spoken. After all there are still humans on Pandora and Jake still has to interact with them. He won't 'completely' go Native, but will definitely speak more Na'vi.
I was also thinking, that with his tsa'heylu with Neytiri he would have picked up more of her intuitive knowlege of the language, and I can hardly wait to see if his passing through Eywa had an effect on him as well.